Comparison of influenza vaccine effectiveness using different methods of case detection: clinician-ordered rapid antigen tests vs. active surveillance and testing with real-time reverse-transcriptase polymerase chain reaction (rRT-PCR).

Annual evaluation of influenza vaccine effectiveness (VE) is needed to assess ongoing impact of immunization efforts in the setting of antigenic drift and periodic vaccine reformulation. Optimal methodology for determining VE remains unclear. We compared influenza VE generated from prospective enrollment and rRT-PCR testing (active surveillance group) with VE based on clinician-ordered diagnostic tests (clinical testing group) in a defined population over four seasons. VE was calculated as (1 - adjusted OR) for vaccination in cases vs. test-negative controls. VE based on clinical testing underestimated VE based on active surveillance and testing with rRT-PCR by 5-33% depending on season.